1. Field of the Invention
The present invention relates to a water-in-oil (W/O) emulsion containing ascorbic acid, to the use of this emulsion for topically treating the skin, on the face, including the area around the eyes, on the body and on the scalp of human beings, as well as to a process for treating the skin topically using this emulsion.
2. Description of the Background
It has been sought for a long time to stabilize ascorbic acid, or vitamin C, in appropriate pharmaceutical presentations, on account of its beneficial properties. This is because ascorbic acid has many biological functions, such as the stimulation of collagen synthesis, the strengthening of skin tissues against external attacking factors, depigmentation, its anti-free-radical activity, and the compensation for vitamin E deficiency.
However, on account of its chemical structure (.alpha.-keto lactone), ascorbic acid is very sensitive to the influence of environmental parameters such as light, oxygen and water. This results in an unavoidable degradation over time of ascorbic acid in solution form leading to a loss of its activity. Moreover, degradation, even slight, of the ascorbic acid, causes yellowing of a composition containing it.
To reduce or delay the degradation of ascorbic acid in solution form, it has been recommended in U.S. Pat. No. 5,140,043 to introduce it into aqueous-alcohol solutions, formed of at least 80% water and having a pH of less than 3.5. However, repeated application of solutions of strongly acidic pH to the skin can disrupt the skin's equilibrium and in particular cause skin irritation.
Moreover, EP-A-670,157 describes the stabilization of vitamin C in a W/O emulsion having a pH of not more than 3.5 and containing, as emulsifier, a dimethicone-copolyol and/or an alkyldimethiconecopolyol. Such a W/O emulsion is better tolerated by users than an aqueous-alcoholic solution, since the aqueous acidic phase containing ascorbic acid is applied in small amount to the skin, in the form of fine droplets dispersed in the oil, which causes no skin irritation or burning. However, repeated application of a composition of acidic pH can have drawbacks for individuals with sensitive skin.
In addition, WO-A-95/28092 discloses stabilizing active agents, such as enzymes or vitamins, in a water-in-oil emulsion containing a high concentration of polyols and an emulsifier chosen from monoglycerides and polyglyceryl ricinoleate. However, the emulsions described in this application contain less than 10% water, and there is still a need for emulsions which can contain more water while at the same time maintaining good stability of the ascorbic acid.
While, it is Inown that the pH has an influence on the stability of ascorbic acid. Thus, the article by B.R. Hajratwala entitled "Stability of ascorbic acid", published in Revue Sciences Pharmaceutiques on Mar. 15, 1985, shows that the decomposition of ascorbic acid exhibits minima at pH 2.5-3 and at pH 6. However, all the tests presented in that article are carried out using simple solutions of ascorbic acid in water and a person skilled in the art cannot deduce therefrom the behavior of ascorbic acid in an emulsion, in which the interactions of the oils and the surfactants modify the environment appreciably and unpredictably. In addition, prior literature in the cosmetics field encourage those skilled in the art to use an acidic pH (see U.S. Pat. No. 5,140,043 and EP-A670157, mentioned above).
The need thus exists for a composition which can be used in the cosmetics and/or dermatological fields, in which ascorbic acid is stable and which causes no skin irritation after application, even for individuals with sensitive skin.